The DART mission to nudge an asteroid

Marc_G

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Of course! That's the easy part. But if we nudge the asteroid a small bit, it might have an effect on an asteroid it passes within within several hundred miles of a few years from now. Then that asteroid (#2) now has a slightly different orbit/movement around the sun/Jupiter/asteroid #3. Now, asteroid #3's movements are now modified so that in 50 years, it collides with asteroid #4. This results in asteroid #4 turning into asteroids #5-15. And one of othose new asteroids ends up having a trajectory that intersects with Earth's orbit 75 years from now.

Far fetched? Sure! But are you telling me that the mission planners looked at every single one of those possibilities and concluded that wouldn't happen? I doubt it. For one thing, they don't have the time or money. But more importantly, they can't calculate a domino effect when they aren't even sure how many dominoes there are, let alone how big they are, what their mass is and what their orbits are.

My whole point isn't that the above will happen. My whole point is that the above is possible under the laws of orbital mechanics as we understand them because we don't fully understand 100% what will happen when we hit that asteroid and what objects are floating in and around the asteroid we hit.

The asteroid belt is sort of a rock soup. Bits and pieces hit each other with some frequency, altering their trajectories in ways that are, in aggregate, beyond our ability to model.

I will grant you the hyper-theoretical infinitesimal chance that an experiment like DART could lead to a bad consequence very far in the future.

BUT it's just as likely that some unplanned effect of pebble tapping rock tapping boulder would lead to the DIVERSION of an asteroid that would have hit us many years from now. This cancels out the tiny unpredictable risks you are hyping. You missed this factor in your logic.

This brings us back to: we are not safe from asteroids. This experiment does not directly measurably increase or decrease risk of us getting hit. It does give us data that can lead to tools we can use against a one-day-identified threat. In this way, the effort is a net positive for us.
 

mh9162013

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You missed this factor in your logic.
I didn't miss that factor, as my overall point wasn't that the DART mission was dangerous. Rather, my point was the reasoning for justifying the mission was also support for the danger inherent in the mission.

So yes, there's a chance the mission could save earth as much as harm earth (from the "domino" perspective). But that wasn't my point. My point was that we can't say something is certain when the whole reason we're doing it is beccause we're not sure how it works.

For the record: I agree 100% ;) that the positives of this mission far outweigh the negatives.
 

Marc_G

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When watching the NASA livestream, did anyone else notice that the last few pictures gradually came slower and slower? This confused me at first since the earlier pictures came at a steady pace. I eventually realized that the processing time for the images got longer as they had more data and weren't just "mostly black space" but rather images of the rocks with ever increasing detail. Cool stuff.
 

Sooner Boomer

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To build off what @Mushtang
[snip]
Now let's assume Musthang is wrong because, as many of you have already mentioned, it'll tell us things we don't know. But that increase in knowledge implies our understanding of the mission is incomplete. And that means that the mission could have consequences we don't understand. And if the impact could produce results we don't understand, it also means it can product results we don't expect. And we don't expect this impact to lead to some other hunk of rock getting slightly modified in its trajectory/orbit so that it's now headed toward earth when it previously wasn't, right? RIGHT?

The mission was an experiment. We did not know the outcome ahead of time. If we had, it would not be an experiment, it would be homework.
 

OverTheTop

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You can't have it both ways. You can't say, "we need to do this mission b/c simulations and math can't tell us exactly what happens if we hit the asteroid. But we know with 100% certainty that nothing bad will happen from the mission."
Of course you can. Don't confuse "We are not absolutely certain about what will happen when we impact the asteroid" with "We don't know if we will deflect this asteroid into Earth's path".

The asteroid was chosen deliberately as it doesn't have an orbit that intersects Earth's orbit. Deflecting the smaller of the two in the pair still leaves the pair orbiting on the same orbit for aeons. It is likely that other celestial effects will perturb their orbit more that what we have just achieved. Either way there are no conjunctions with Earth in the foreseeable future.

Note that many years ago there were missions proposed to deflect asteroids coming close to Earth. This was because it would be easy to determine how much the orbit had been deflected. They were never executed due to the possibility that they could incorrectly deflect the asteroid closer to Earth. The binary asteroid was chosen for the DART mission because it could never hit Earth and yet was still able to measure the orbit change with a good accuracy.
 

Funkworks

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I just looked up what's the smallest detectable (known) asteroid, and it seems to be about 6 feet big. If they find more of those, they could play target practice forever and the worst that could happen would be more shooting stars down here on Earth. A 6-foot meteor is not a threat, it would burn up like a used satellite.

 

ernest malleyscrub

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FAQ : The trajectory of DART was frontal collision not side on of with the orbit - wondering why this was chosen? Likely discussed elsewhere(?) Would side on glance risk controversy ( as evidenced by many panic questions even here) ? I would have thought DART pushing impact in direction of orbit would be easier? Maybe the doom panic crowd had a say? Thanks everyone.

Amazing effort to collide with small thing so immensely far away..... Science!
 

LithosphereRocketry

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FAQ : The trajectory of DART was frontal collision not side on of with the orbit - wondering why this was chosen? Likely discussed elsewhere(?) Would side on glance risk controversy ( as evidenced by many panic questions even here) ? I would have thought DART pushing impact in direction of orbit would be easier? Maybe the doom panic crowd had a say? Thanks everyone.

Amazing effort to collide with small thing so immensely far away..... Science!
My guess is just that side-on wouldn't be very effective. If you hit side-on, you put a lot of energy into spinning the asteroid, which doesn't change its orbit. If you hit it head-on, all that energy goes into pushing the asteroid instead of spinning it, which does change its orbit.
 

OverTheTop

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FAQ : The trajectory of DART was frontal collision not side on of with the orbit - wondering why this was chosen?
Orbital mechanics. If you hit it head-on you get the best change in velocity and that translates to a lower orbit and a faster orbit time. Both of those can be measured with reasonable precision relatively easily. If you hit it and try to push it into the companion asteroid there is no nett change in orbital period and you have probably only changed the apoapsis and periapsis, and the change would be very hard to measure with meaningful accuracy IMHO.

I will give them bonus points if they hit it at apoapsis where the energy gives the greatest change in orbital parameters.
 

Marc_G

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FAQ : The trajectory of DART was frontal collision not side on of with the orbit - wondering why this was chosen? Likely discussed elsewhere(?) Would side on glance risk controversy ( as evidenced by many panic questions even here) ? I would have thought DART pushing impact in direction of orbit would be easier? Maybe the doom panic crowd had a say? Thanks everyone.

No doom panic crowd influence, just good experimental design. The experiment being done was really to look at momentum transfer during collision and how that translates into shifting a big chunk of rock that might really just be an aggregated pile of rubble. In this case, a more or less head-on collision that could slow the orbital period of that tiny asteroid moon by a small but measurable amount (modeled at a 10 minute shift in a 11 hour orbital period, easily measurable) is the best way to gather the data of interest.

But, there's more than that when we consider the practical application. There was a time when I was particularly interested in this asteroid deflection stuff, and at that time (years ago) I read up and worked through a bit of the math. Some key learnings I came away with in terms of actually using robotic impactors to prevent an asteroid colliding with Earth:

1. Time is our friend. The longer advance notice we have, the earlier we can tweak the orbit of the asteroid, and the less of a tweak is needed to miss Earth.

2. Think of the Earth as a 7900 mile wide target, moving in its orbit around the sun at about 18.5 miles per second. This boils down to the Earth moving one Earth diameter every 7.1 minutes. So, for any given trajectory of asteroid heading toward earth (more or less within the ecliptic plane) the Earth will only be "there" for 7.1 minutes. If course, this assumes math of where the asteroid is falling directly in. More practical trajectories have it crossing at an angle, that either prolong or shorten the time where Earth is in the crosshairs, depending on the relative directions involved. But, on average, 7.1 minutes.

3. You can either deflect the asteroid (up, down, or to either side) to miss where Earth will be at that time, or you can slow down or speed up the asteroid so that it reaches earth's orbit a bit too early or too late to hit us.

4. If the asteroid was on "final approach" so to speak, the only practical way to reach it is pretty much head-on on a (gravitationally curved) line between earth and the asteroid. It wouldn't be energetically favorable to loop behind it and hit it from the back, or to the side or top or bottom. Any of those maneuvers would waste precious delta-V.

5. If the asteroid is years out, even decades out, we have a lot more options for various trajectories that could impact from different angles to tweak it in different directions, but generally you get the most bang for the buck with something that is essentially head on, slowing the asteroid. If the original trajectory is dead-on-center, you only need to tweak it by 3.55 minutes which is a very small change over a timespan of years.

This is all very neat stuff.
 
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OverTheTop

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There are other orbital effects that can affect the impact too. Looking at Apophos, which was a possible conjunction at the end of the decade, it had been determined that there was a gravitational "keyhole" that if the rock went through it would perturb the orbit and set up a harmonic return to Earth, including impact, two years later. The keyhole was only about 600m across and if it were kept out of there then the harmonic return wouldn't occur.

Also keep in mind that this recent mission is only an experiment. In a real situation there would be a fleet of similar impactors to provide enough energy and redundancy in the attack.
 

Funkworks

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Here's a short video announcing the success:



My guess is just that side-on wouldn't be very effective. If you hit side-on, you put a lot of energy into spinning the asteroid, which doesn't change its orbit. If you hit it head-on, all that energy goes into pushing the asteroid instead of spinning it, which does change its orbit.

+1 to what others said, but also: by aiming for the center, you increase the chances of a hit, which matters when you're gambling over $300M on live TV.
 
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Funkworks

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Good Lord!

A 32 minute change instead of the expected 10?

That means it’s going to hit the Earth!

😱

2bpdap.jpg

Um ... no.

"In a media briefing, NASA Administrator Bill Nelson confirmed that DART successfully changed Dimorphos’ trajectory, changing the orbit around its parent asteroid, Didymos, from 11 hours and 55 minutes to 11 hours and 23 minutes."

 
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boatgeek

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I’m actually now disappointed. I was hoping to an early end to campaign season by global apocalypse.
Living in Wisconsin (or a few other heavily contested states) can't be a lot of fun right now.

Getting 3x the expected change in orbital period is going to make everybody scratch their heads. It seems from some of the reporting that the first thought was that the ejected material caused more change than expected/there was more ejected material. Of course, when you have to completely re-work the model, it may become less accurate when it's really needed.
 

Charles_McG

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Living in Wisconsin (or a few other heavily contested states) can't be a lot of fun right now.

Getting 3x the expected change in orbital period is going to make everybody scratch their heads. It seems from some of the reporting that the first thought was that the ejected material caused more change than expected/there was more ejected material. Of course, when you have to completely re-work the model, it may become less accurate when it's really needed.
During the press conference, they said that while 10 minutes was the best estimate, the model range was from 2 to ‘tens of minutes’, and that 32 wasn’t outside the model range.

From earlier interviews, I gather that they didn’t expect dimorphos to be as fluffy as Bentu or Ryugu because it was a different class of asteroid. The big effect , I think, means that is was that fluffy. Maybe because it was a moon and not the primary? I know they expect some asteroids to be solid rock (looking forward to the psyche mission) - but we’re still learning about the details.
 

Funkworks

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there's no other asteroids coming close for a long time
None that they know of. Risk of collision is determined for each one that is tracked and the risk is low for all of them. I don't know how often they discover new ones.

Living in Wisconsin (or a few other heavily contested states) can't be a lot of fun right now.

Getting 3x the expected change in orbital period is going to make everybody scratch their heads. It seems from some of the reporting that the first thought was that the ejected material caused more change than expected/there was more ejected material. Of course, when you have to completely re-work the model, it may become less accurate when it's really needed.
I think anything above a 10min change was to be considered a success. The 32min they got can be seen as a measurement they were looking for: they "discovered" that such and such impact conditions resulted in a 32min change. That's valuable data to understand how strongly asteroids are held together. 3x looks like a lot, but it's still a small fraction of a 11h 23min orbital period (~ 4.5%).

From earlier interviews, I gather that they didn’t expect dimorphos to be as fluffy as ...
Yeah, being able to distinuish a fluffy asteroid from a rocky one sounds important to set future impact conditions.
 
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boatgeek

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During the press conference, they said that while 10 minutes was the best estimate, the model range was from 2 to ‘tens of minutes’, and that 32 wasn’t outside the model range.

From earlier interviews, I gather that they didn’t expect dimorphos to be as fluffy as Bentu or Ryugu because it was a different class of asteroid. The big effect , I think, means that is was that fluffy. Maybe because it was a moon and not the primary? I know they expect some asteroids to be solid rock (looking forward to the psyche mission) - but we’re still learning about the details.
That is really helpful context.
Yeah, being able to distinuish a fluffy asteroid from a rocky one sounds important to set future impact conditions.

Clearly that "rock" was actually made of fluffy materials, or simply not as dense as they suspected.
If we ever have to do this for realsies to protect the planet, it might be a matter of scheduling hits over a period of time (a week? Two? Probably depends on launch capacity to actually get the impacters flying) If the initial strike shows a lot of fluff blown out, then the calcs could be updated to see if all of the strikes are needed/wanted. If not, the last few could be diverted away from the asteroid.
 
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